Oil production optimization and enhanced recovery method and apparatus for oil fields with high gas-to-oil ratio

ABSTRACT

A method for optimizing oil production rate from an oil well with high gas-to-oil ratio is disclosed to include modeling an Inflow Performance Relationship curve and calculating an optimal level of bottomhole pressure to be higher than zero. Maintaining the bottomhole pressure at that calculated optimum level by using a bottomhole tool of the invention or other known means such as gas injection provides for maximum oil recovery from a given well. The bottomhole tool includes a multi-stage flow resistor and a needle moved in and out of the resistor by a spring-biased piston responsive to a difference in pressure between a bottomhole pressure and a pipe pressure. Automatic adjustment of the bottomhole pressure is maintained over a wide range of operating parameters throughout the life of the well.

CROSS-REFERENCE DATA

Priority is claimed herein from a U.S. Provisional Application No.60/549,992 by the same inventor, as filed Mar. 05, 2004 and entitled“OIL PRODUCTION OPTIMIZATION AND ENHANCED OIL RECOVERY METHOD ANDAPPARATUS FOR OIL FIELDS WITH HIGH GOR”, incorporated herein in itsentirety by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to a method and devices forincreasing the production of oil. More specifically, the method and thebottomhole tool of the invention provide for maintaining the bottomholepressure at a level optimum for maximizing oil production in a well withhigh gas-to-oil ratio (GOR). The most advantageous are of implementationof the present invention is in wells with high GOR defined as GORgreater than 600 cubic feet per barrel. In these wells the method andthe tool of the invention can be used when the bottomhole pressure islower than the bubble point pressure as well as in all cases when thegas cone has appeared such as in fountain, gas lift, and pump regimes ofoil production.

Optimization of oil production has been a goal of many methods anddevices of the prior art. Generally speaking, the bottomhole behavior ofoil mixed with gas and some other ingredients such as water, etc. hasbeen described in a series of mathematical equations by Muskat. Onespecific publication of Muskat is incorporated herein by reference inits entirety and describes the mathematical model of oil reservoir:Muskat M. “The Production Histories of Oil Producing Gas-DriveReservoirs”, published in the Journal of Applied Physics in March of1945, p.147-159.

For illustration purposes, a one-dimensional axis-symmetrical system ofMuskat equations with corresponding PVT characteristics of fluid anddependencies of relative permeability K_(ro), K_(rg) from liquidsaturation (S_(o)) can be described as follows:${\frac{1}{r}\frac{\partial}{\partial r}\left( {r\frac{k_{ro}}{\mu_{o}B_{o}}\frac{\partial p}{\partial r}} \right)} = {{- 158.064}\frac{\phi}{k}\frac{\partial}{\partial t}\left( \frac{S_{o}}{B_{o}} \right)}$${\frac{1}{r}{\frac{\partial}{\partial r}\left\lbrack {{r\left( {\frac{k_{rg}}{\mu_{g}B_{g}} + {\frac{Rs}{5.615}\frac{k_{ro}}{\mu_{o}B_{o}}}} \right)}\frac{\partial p}{\partial r}} \right\rbrack}} = {{- 158.064}\frac{\phi}{k}\frac{\partial}{\partial t}\left( {\frac{S_{g}}{B_{g}} + {\frac{S_{o}}{B_{o}}\frac{Rs}{5.615}}} \right)}$where: P—pressure in formation; S_(o)—oil saturation in formation;S_(g)—gas saturation in formation; R_(s)—solution of gas in oil;B_(o)—oil formation volume factor; B_(g)—gas formation volume factor;μ_(o)—oil viscosity; μ_(g)—gas viscosity; Φ—formation porosity;K—formation permeability.

For practical purposes, Vogel had simplified the Muskat equations andadapted them to the calculations of oil producing formations. Theseequations are known as Vogel model and have subsequently been modifiedby others. One example of such publication is as follows: Vogel, InflowPerformance Relationships for Solution-Gas Drive Wells, as published inJournal of Petroleum Technology, January 1968, pp. 83-92, incorporatedherein in its entirety by reference. Unfortunately, Vogel model does notwork well in wells with high gas-to-oil ratio. According to Vogel, thedependency of oil rate production of bottomhole pressure is a constantlydiminishing parabolic curve with a production peak at zero bottomholepressure, see for example FIG. 2 of the above mentioned article. Inother words, the lower the bottomhole pressure is, the higher is the oilrate production from the formation. This is a gross simplification ofthe bottomhole processes in the formation. In fact, if the bottomholepressure falls below saturation pressure in case of high GOR, relativepermeability coefficient by oil decreases because of gas saturationincrease, which in turn is a result of gas being released from oil.Viscosity of so degassed oil also increases. This leads to a decrease ofproductivity index of formation. This phenomenon effects the oilproduction rate more than the increasing depression. As a result,decreasing of the bottomhole pressure below saturation pressure can leadto a decrease in oil production rate, rather than to its increase aspredicted by Vogel's model, see FIG. 1. In some extreme cases, relianceon Vogel's model will cause a complete switch in production from oil togas. There is a need therefore for a method allowing calculating the oilproduction rate in high GOR wells with better accuracy then that allowedby Vogel's model.

More specifically, the need exists for a method of calculating wellparameters in an optimal regime that takes into account two opposingprocesses. The existence of this optimal regime is explained by twophenomena simultaneously affecting the current oil rate in two oppositedirections in the skin layer. On one hand, reducing the bottomholepressure (increasing depression in formation) leads to increased oilrate:Q _(oil) ˜K(P,S _(L))·(P _(form) −P _(bottom)),where Q_(oil)—oil rate; K(P,S_(L))=(ko*h)/(mu*Bo)—production index;P_(form)—formation pressure; P_(bottom)—bottomhole pressure; ko—relativeoil permeability; h—length perforation interval ; mu—oil viscosity ;Bo—oil formation volume coefficient; S_(L)—saturation of liquid).

On the other hand, it reduces the production index (K(P,S_(L)), becausegas dissolved in oil comes out of solution, reducing therefore relativeoil permeability of formation. Production index is additionallydecreased due to an increased viscosity of degassed oil, which alsosignificantly decreases oil mobility.

Thus, as the bottomhole pressure is decreasing, at first the oil ratebegins to increase due to the increase of depression in formation. But,beginning with a specific bottomhole pressure (from now on calledoptimal bottomhole pressure), the oil rate starts to decrease eventhough the depression increases further, which is contrary to widelyknown models of Vogel and Fitzkovich. The reason for it is that afterreaching the optimal bottomhole pressure, the influence of decreasingproduction index becomes dominating. This phenomena can be explained bystrong non-linear relationship between the relative oil permeability offormation and its oil saturation for most often used saturation values(S_(L)=0.5÷1.0).

Besides, degassed oil not only becomes more viscous, but also shrinks involume, which together with gas in free form creates a blocking zone,preventing exit of oil from formation and reducing oil saturation here.Strong skin effect may appear in a near bottomhole zone. FIG. 5illustrates this situation, in which the well 100 contains a wellheadchoke 110 at the surface and a bottomhole tool 120 close to thebottomhole formation consisting of saturated oil reservoirs 150, waterlayer 180, and gas layer 170. Note the areas of gas cone 130, water cone140 and viscous barriers of oil with low mobility 160.

As a supplemental consideration, decreasing bottomhole pressure furtherincreases GOR because of increased relative gas permeability offormation. This causes gas to prematurely exit formation, which in turnaccelerates falling of formation pressure and as a result reduces theultimate oil recovery index.

The presence of a point of flow rate maximum on the IPR curve (and thusthe optimal bottomhole pressure) may also be explained by presence ofgas and/or water cones, which reduce the active oil inflow perforationinterval, and expand the segments surrounded by gas and water cones,appearing and growing when the bottomhole pressure decreases. GOR alsosignificantly increases in that case. FIG. 9 demonstrates a visible peakin oil rate on an actual IPR curve obtained from an oil well in a largeSiberian oil field. The maximum oil flow rate is observed at abottomhole pressure not equal to zero.

A further need exists for a bottomhole tool allowing adjustments inbottomhole pressure in a well. Many designs of bottomhole tools andmethods of controlling the bottomhole pressure are known in the priorart. One of such devices is disclosed in U.S. Pat. No. 5,105,889. Thisdevice includes a set of axially vertically aligned pipes of differentdiameters and lengths, forming a multi-parameter hydrodynamic system.That system establishes a certain pre-calculated bottomhole pressurebelow the device, in order to decrease gas blockage of the nearbottomhole zone of the oil formation and to provide a stable fluid flowto the surface. A forced fluid degassing takes place in the device,creating a two-phase gas-liquid emulsion in order to provide asufficient fluid lift within the well. The device disclosed in thispatent has however certain limitations. A pressure differential acrossthe device depends on the calculated diametrical parameters of thepipes. That in turn corresponds to current values of the flow parametersin the formation. Such fixed dependency restricts the adaptability ofthe device to changing reservoir and well conditions.

Another method and device is disclosed in the U.S. Pat. No. 5,752,570.In accordance with this patent, the bottomhole pressure is automaticallymaintained higher than a current saturation pressure of the formationfluid with gas in the near bottomhole zone of the formation, regardlessof fluctuations of fluid pressure in the formation. This is done inorder to create fluid flow with minimum gas content. Once the bottomholepressure decreases, the device automatically creates conditions forformation of a fluid flow into the device with an increased speed.Nearly mono-phase flow is transformed within the device into a finelydispersed gas-liquid two-phase flow, in order to provide its lift to thewellhead. The device disclosed in this reference automatically adjustsbottomhole pressure to a desired level, simultaneously providing apressure drop, in order for the fluid to sustain degassing within thetransforming area, according to the device inlet pressure at thebottomhole. However, in the process of oil field development,operational conditions change as well as the inflow performance curvecorresponding to a current well operation. The sensing element of thedevice disclosed in this reference might no longer maintain the sameoptimal well operation, since its calibration is based on the previouswell information parameters. Besides, calculations have proven that insome wells a space between the inner nozzle surface and the outersurface of the regulating cone of the device reduces to approximately0.01 inch. With such a small space even a trace of sand in the fluid canjam the regulating unit and stop the well production. Since the pressuredifference depending on the movement of the regulating cone has anon-linear characteristic and is a function of fixed power of thediameter of the adjustable cross-section, it impedes preciseregulations.

A further example is disclosed in the U.S. Pat. No. 5,967,234incorporated herein in its entirety by reference. Means forautomatically adjusting the bottomhole pressure are described in thispatent to include a spring-biased needle traveling inside a plurality ofpipes of diminishing diameters. The space left between the needle andthe corresponding pipe is available for oil flow and can be adjusteddepending on the bottomhole pressure. Fixed geometry of the needle andthe pipes makes this device limited in its field of use as changingparameters of the well require a broader range of adjustment of flowrestriction then this device can provide.

The need exists therefore for a method and device with broad range ofparameters that can be adjusted preferably from the surface of the wellto bring the bottomhole pressure in agreement with the required valuesto maximize the production of oil from an oil well.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to overcome theseand other drawbacks of the prior art by providing a novel device andmethod for optimizing and maximizing the production of oil from an oilwell, particularly an oil well with high GOR.

It is another object of the present invention to provide a methodallowing calculating and maintaining the optimum value of bottomholepressure required to maximize oil production and operating life durationof the well.

It is a further object of the present invention to provide a bottomholetool allowing adjustment of bottomhole pressure from the surface in awide range of formation conditions and throughout the life of the wellwithout the need to replace the device.

It is yet a further object of the present invention to provide abottomhole tool allowing adjustments of bottomhole pressure in a desiredrange such that the reliability of that tool is increased by providinglarger values of clearances between the moving and non-moving parts ofthe tool. Increased reliability would depend on the resistance of thetool to jamming by sand and other particles present in oil flow.

The method of the invention is based on a mathematical model taking intoaccount and accounting for all four key elements of oil production,including reservoir model, poly-phase flow in pipes, flow through thebottomhole tool and flow through the surface choke. The mathematicalmodel of the method of the invention allows calculating the optimumvalue for bottomhole pressure so that the oil rate production ismaximized. Characteristics of all four elements are entered continuouslyinto the equations and allow calculating and adjusting the value ofbottomhole pressure throughout the life of the well and in variousoperating conditions thereof.

A multi-parameter bottomhole tool with flexible characteristic ofpressure regulation is also proposed with a broader range of adjustmentsof the operating parameters then in the previously known devices. Thisis achieved by novel modifications of the tool's geometricalcharacteristics, i.e. by using of several sections with predeterminedlengths and cross-sectional areas to create the noncircular channel forpassing the fluid. The tool includes a series of pipes with decreasingdiameters and a corresponding multi-stage piston- or spring-biasedneedle with diameters of stages selected to correspond to that of thepipes. Longitudinal movement of the needle along the length of thedevice allows changing of a greater number of parameters affecting theperformance of the tool and therefore broadens the range of operation.This allows expansion of dynamic ranges of the controlled pressure dropand the fluid velocity without replacement the tool.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the subject matter of the presentinvention and the various advantages thereof can be realized byreference to the following detailed description in which reference ismade to the accompanying drawings in which:

FIG. 1 is an inflow performance relationship curve according to Vogeland according to the present invention,

FIG. 2 is a sample PVT data needed for the method of the presentinvention,

FIG. 3 is a sample chart showing relative permeability of oil and gasversus liquid saturation,

FIG. 4 is a cross-sectional view of the bottomhole tool of the presentinvention,

FIG. 5 is an illustration of the negative effects in the near bottomholezone of the formation,

FIG. 6 is a mathematical model chart showing the formation pressure, oilrate and GOR curves as a function of oil recovery,

FIG. 7 illustrates a mathematically modeled well performance in a givenperiod of time,

FIG. 8 is a mathematical model of a sample IPR curve, and

FIG. 9 illustrates the actual IPR curve with a peak oil recovery ratevisible on the chart.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The main concept of the method of the present invention lies in thediscovery that there exists an optimal level of bottomhole pressureallowing to maximize the oil production rate and that this optimalbottomhole pressure does not necessarily have to be the lowestbottomhole pressure of the formation.

The method of the invention is based on an integrated mathematical modelof the production process incorporating the following four keycontributing factors defining the oil production: formation, multi-phaseflow through pipes, surface choke flow and bottomhole tool flow.Calculations of these four factors will be described in more detailbelow.

FORMATION CALCULATIONS

First of all, according to the invention, basic Muskat equationsdescribing the bottomhole formation and behavior of various parametersduring the oil production operation are transformed in a way differentfrom that of Vogel. Muskat equations were initially picked as amathematical model, which describes basic processes of unsteadytwo-phase filtration in formation; with some simplifying assumptions asfollows:

-   -   formation is one dimensional and there exists only radial flow;    -   porous media is isotropic and uniform;    -   gravity and capillary effects can be neglected;    -   compressibility of rock and water can be neglected;    -   constant pressure exists in both oil and gas phase.

These assumptions make it possible to describe the two-phase flow of oiland gas by the partial differential equations as follows:$\quad\left\{ {\quad\begin{matrix}{\quad{{\frac{1}{r}\frac{\partial}{\partial r}\left( {r\frac{K_{ro}}{\mu_{o}B_{o}}\frac{\partial P}{\partial r}} \right)} = {{- 158.064}\frac{\varphi}{K}\frac{\partial}{\partial t}\left( \frac{S_{o}}{B_{o}} \right)}}} \\{{\frac{1}{r}\frac{\partial}{\partial r}\left( {{r\left( {\frac{K_{rg}}{\mu_{g}B_{g}} + {\frac{Rs}{5.615}\frac{K_{ro}}{\mu_{o}B_{o}}}} \right)}\frac{\partial P}{\partial r}} \right)} = {{- 158.064}\frac{\varphi}{K}\frac{\partial}{\partial t}\left( {\frac{1 - S_{o} - S_{w}}{B_{g}} + {\frac{S_{o}}{B_{o}}\frac{Rs}{5.615}}} \right)}}\end{matrix}} \right.$

Zero flow condition on the outsize border of the zone is:$\quad{{\frac{\partial P}{\partial r}❘_{r = r_{e}}} = 0}$

On the wall of the well, a border condition is set based on known valueof pressure or oil rate:${P❘_{r = r_{w}}} = {{{{P_{w}(t)}\quad{or}\quad\frac{\partial P}{\partial r}}❘_{r = r_{w}}} = {F_{w}(t)}}$

Initial conditions are also set as follows:P(r,t)=P ₀(r,0); S(r,t)=S ₀(r,0);

The above equations can be computed with available PVT data usuallypresented as a chart such as shown for example on FIG. 2 as well astaking into account the dependence of relative permeability of differentphases from saturation (as shown for example on FIG. 3) and with thefollowing other properties of reservoir: μ_(o)(P), μ_(g)(P), B_(o)(P),B_(g)(P), R_(s)(P), K_(o)(S_(o)), K_(g)(S_(g)), K, Φ, P_(f), P_(bp),r_(w), r_(i), S_(w), and S_(g crit).

MULTI-PHASE FLOW THROUGH PIPES

A second component of a mathematical model consists of a number ofmathematical equations describing the flow of gas-oil-water mixture(depending of course on the specifics of each individual well) through asystem of pipes connecting the bottomhole area of formation to thesurface. In a typical scenario, this is a multi-phase flow system ofequations. They are well known in the art and can be found in thepublications by Aziz. One such publication is Aziz K. et al. PressureDrop in Wells Production Oil and Gas, Journal of Canadian PetroleumTechnology, 1972, incorporated herein in its entirety by reference. Over30 input fluid parameters are needed for these calculations, which arecollected prior to running the model.

SURFACE CHOKE FLOW CALCULATIONS

Gilbert's model was used for simulation of the multi-phase flow of thesurface choke. It is known in the art and can be found in the followingpublication incorporated herein in its entirety by reference: ArtificialLift Methods, Volume, ed. Kermit E. Brown. Main input parameters includeP1 and P2 as input and output pressures; GOR—gas-to-oil ratio; D—chokediameter; Q—oil flow rate.

BOTTOMHOLE TOOL DESCRIPTION AND FLOW CALCULATIONS

A detailed description of the device of the present invention followsnow with reference to accompanying drawing on FIG. 4 in which likeelements are indicated by like reference letters numerals.

The bottomhole tool of the invention is mounted in a well 10 at the endof the pipe 15 sealed to the well 10 through the sealing ring 11. Thehousing 20 of the tool is attached to the lower end of the pipe 15 byany known means such as by a threaded connection as shown on thedrawing. A multi-stage telescopic fluid resistor 30 is attached to thelower portion 21 of the housing 20 and contains cylindrical stages 31,32, 33, and 34 having diameters decreasing toward the bottom of thedevice. Although the drawing shows four such stages, it should beunderstood that any number of stages starting with just two stages iscontemplated by the present invention. Provisions are made to directsubstantially all fluid flow into the central inside portion of thetelescopic fluid resistor 30 through a tapered opening at the bottom ofthe lower portion 21 of the tool housing 20.

A multi-stage needle 40 is located inside the telescopic fluid resistor30 and consists of several stages 41, 42, 43, and 44 having diametersincreasing in the direction toward the bottom of the tool. Thesediameters are chosen in such a way that they are all smaller then thediameter of the smallest stage 31 of the resistor 30 so that the needlecan travel up and down the entire length of the resistor 30. Preferably,the difference between the largest stage 41 of the needle 40 and thesmallest diameter 31 of the resistor 30 is sufficient enough for passingsand and other inclusions so as to prevent well clogging duringoperation. Exact diameters and lengths of the various stages of theneedle 40 and the resistor 30 are calculated from the mathematical modelas described herein. Preferably, the ranges of diameters for the needle40 are between about 1 and about 50 mm, preferably between about 3 andabout 20 mm and for the resistor 30 these diameters are between about 2and about 55 mm, preferably about 4 to about 25 mm. It is also preferredto have the lengths of various stages of the needle 40 correspond tothat of the resistor 30. In that case, the flow calculations are welldefined to the series of several successive annular passages ofwell-defined lengths, at least at the lower position of the needle 40.

The needle 40 is supported by and moved up and down as a result of itbeing connected to a pressure-responsive means consisting of the activepiston 51 of the control cylinder 50 responsible for automatic pressureadjustment in the bottomhole tool of the present invention. The housing56 of the control cylinder 50 is attached to the lower part 21 of thetool housing 20 and is sealed at the bottom. Inside the housing 56 thereis located the piston 51 supported by a spring 52 and exposed to twopressures. The first pressure above the piston 51 is that of thebottomhole formation P1, as transmitted through an opening 55. Thesecond pressure is that which acts below the piston 51 and is a pipepressure P2, as transmitted through a small diameter pipe 53 and theopening 54. The motion of the piston 51 is therefore determined by apressure differential P2-P1 and the compression of the spring 52. Thelength of the cylinder 56 is chosen to provide for enough stroke lengthfor moving the needle 40 along the operating range of the resistor 30.

In the beginning of the operation of the bottomhole tool of theinvention, the needle 40 is completely introduced inside the resistor30. In some cases it can be partially introduced, and in other cases itcan be completely withdrawn from the lower portion of the resistor 30,depending on the well and formation conditions. After installation ofthe device and starting of the well, the phase oil permeability, in thenear bottomhole zone of the reservoir increases and as a result of that,the oil flow rate increases. In response, the pressure differentialwithin the device grows. The piston 51 is displaced in the cylinder 56,and in turn it displaces the needle 30 downwards. The piston 51 is undera pressure differential P1 minus P2. The position of the piston 51 isbalanced by the spring 52 such that the initial movement of the piston51 connected with the needle 40 starts only when a force generated bythe pressure differential exceeds a force of the pre-compressed spring52.

Before any movement of the piston 51 initiates, the pressuredifferential within the device corresponds to the initial hydraulicresistance, with the needle 40 seated fully inside the fluid resistor30. As the oil flow rate reaches a certain point, its further growth maycause an extremely rapid increase of pressure differential within thedevice, so the needle 40 starts to pull down from the resistor 30. Thebalancing force of the spring 52 stops the downward movement such thatthe hydraulic resistance of the device is reduced and the bottomholepressure is again maintained at a desired level.

When the cylinder needle 40 is completely pulled out of the resistor 30,the hydraulic resistance of the tool is minimal. Such resistancecorresponds to a resistance of a system of telescopic pipes having around cross-section. The pressure differential within the device inresponse to a further increase of flow rates will be based on a constant(minimal) hydraulic resistance of the lower stage 31 in addition to thenext stage 32 and finally to further stages 33 and 34. If the flow ratesdecrease due to some changes in the reservoir and fluid parameters andreduction of the reservoir pressure, the needle 40 will start movingback up into the body of the resistor 30. This in turn adjusts thehydraulic resistance of the tool to a desired optimum level in order tomaintain optimum bottomhole pressure and maximum oil flow ratesaccording to the current conditions of the formation, reservoirpressure, and fluid parameters.

Due to the above described self-regulation of the tool, the device ofthe present invention can operate efficiently in a wide range offormation, reservoir, and fluid parameters, all varying with time,without the necessity to remove the device from the well. Morespecifically, formation parameters change during the operation of awell, such as formation pressure, gas, oil and water saturation, phasepermeability as well as such fluid parameters as water-oil and gas-oilratio, viscosity, surface tension, etc. With prior art systems, it wasnecessary to replace the bottomhole equipment in the well with a newequipment having characteristics corresponding to the current formationand fluid parameters. With the method and device in accordance with thepresent invention no replacement of the bottomhole equipment is needed.The tool of the invention automatically maintains the desired bottomholepressure of the formation fluid at the level needed for maintaining themaximum flow of the formation fluid from the bottomhole of the well tothe surface wellhead. The device in accordance with the presentinvention provides automatic adjustment of its parameters in response tothe changing formation parameters and fluid properties.

An increased differential pressure between the formation and thebottomhole pressure usually results in increased oil flow rates.However, in formations with high gas-oil ratio, a decrease in bottomholepressure causes formation oil degassing in the near bottomhole zone ofthe formation, increase in oil viscosity, reduction of the formation oilpermeability and as a result, reduction of the formation productivity.Further reduction of bottomhole pressure may result in a decrease of oilflow rate rather than its increase. The optimum pressure will change intime according to change of parameters of fluid and formation.Maintenance of an optimum bottomhole pressure by means of the inventivedevice in the formations with gas and water coning provides for themaximum oil flow rates with minimum gas and water flow rates.

The following publications contain mathematical equations used tocalculate the flow through the bottomhole tool of the invention, all ofwhich are incorporated herein in their entirety by reference:

-   -   Two-phase flow in vertical noncircular channels, International        Journal of Multiphase Flow, vol. 8, 1982, pp 641-655;    -   Sudden Contraction Losses in Two-phase Flow., Journal of Heat        Transfer, February 1966; and    -   Some Characteristics of Gas-Liquid Flow in Narrow Rectangular        Ducts, International Journal of Multiphase Flow, vol. 19, No. 1        ,1991, pp. 115-125.

The method of the invention consists therefore of several steps indefining and maintaining the optimum level of bottomhole pressure inorder to maximize oil production:

-   -   a) collecting formation and oil well input data, such as on the        current conditions of the well, bottomhole zone, fluid and        reservoir parameters, PVT, geometry and dimensions of pipes,        bottomhole tool and a wellhead surface choke and so on to        populate the mathematical model describing        “formation—multi-phase flow—surface choke—bottomhole tool”        behavior;    -   b) modeling or simulating the entire Inflow Performance        Relationship curve describing the relationship of the bottomhole        pressure and the oil production rate similar in general to that        shown on FIG. 1 but specific to a particular well;    -   c) calculating the desired higher than zero value of the        bottomhole pressure from the IPR curve as calculated in step        (b);    -   d) adjusting the bottomhole pressure to the vicinity of the        desired level corresponding to current well conditions by any        number of available means including performing a gas lift,        adjusting the bottomhole choke of the generally known design or        inserting an appropriately sized bottomhole tool of the        invention;    -   e) in case the bottomhole tool of the invention is used,        conducting final adjustment of the bottomhole pressure by        adjusting the wellhead surface choke and thereby the pressure        above the bottomhole tool of the invention;    -   f) starting oil fluid flow and monitoring well parameters to be        within the desired levels to ensure maximum oil flow rate as        well as compare the actual flow rate to that predicted by the        model, adjust the model if necessary;    -   g) if deviation of the well parameters is detected,        recalculating the optimum bottomhole pressure and adjust it        according to newly calculated value using the previously        described steps;    -   h) maintaining the bottomhole pressure at the optimum level so        that the oil flow rate is maximized throughout the life of the        well or the operation of the device of the invention.

EXAMPLE OF USING THE METHOD OF INVENTION

As an example, the following formation was analyzed and mathematicalmodel was calculated for: radius R_(f)=1000 ft; height H=50 ft; Φ=0.15;K=15 μD, r_(w)=0.3 ft, with PVT characteristics shown on FIG. 2 andfunctions K_(ro)(S_(L)) and K_(rg)(S_(L)) shown on FIG. 3. Extractionmethod was regime solution gas. Illustrative data, results and chartsare shown on FIGS. 6-8.

The resulting three cases of solution are shown on FIG. 6:

-   -   Case I—the case when bottomhole pressure was kept throughout the        life of the well at a non-optimal level of        P_(bot)(t)=0.25.P_(f)(t);    -   Case II—the case when bottomhole pressure was kept throughout        the life of the well at an optimum level of P_(bot)(t)=P_(bot)        ^(opt)(t); and    -   Case III—the case when at first for approximately 120 days the        well worked according to scenario as in case I, and then it was        switched to scenario as in case II.

Behaviors of oil rate (Q_(oil)), formation pressure (P_(f)), and GOR, independence of current recovery index (N) are shown on FIG. 6 aspredicted by using the calculations according to the method of thepresent invention. In case I, the well worked for approximately 990 daysbefore the oil rate fell to 6 bar/day, the limit of productionsensibility. By that time, the well gave approximately 4.25% of theultimate recovery index. In the second case, the well worked for 1440days (4 years), and gave approximately 9.8% of the ultimate recoveryindex, more than double that of the first case. In case III (see FIG.7), when the well was switched to optimal regime 120 days afterproduction started, the ultimate oil recovery index increased from about4.25% to about 6.2%. At the same time, switching the well into optimalregime reduced GOR and increased oil rate from 130 bar/day to 250bar/day. The lifetime of the oil well in that case is increased to about3.4 years.

All these desirable effects were achieved due to keeping the bottomholepressure at the optimal higher than zero level, which caused reductionof forming of oil blocking zone in formation near bottomhole and sloweddown loss of gas from formation, which in turn may cause formationpressure to drop. FIGS. 6 and 7 also illustrate that maintaining thebottomhole pressure at the optimum level as calculated using the methodof the invention substantially increases the ultimate oil recovery froma given well.

FIG. 8 shows a calculated IPR curve for an oil well with formationparameters amenable to using the method of the present invention. Thepresence of the optimum value of the bottomhole pressure is seen whichis not equal to zero. That bottomhole pressure corresponds to themaximum oil production rate for these formation and oil well conditions.Also of note is the strong tendency of GOR to increase with bottomholepressures falling below the optimum level.

Besides the obvious benefit of increasing the oil flow rate and oilrecovery index from the well, the method and device of the inventionprovide for the following important advantages:

-   -   reduce gas-to-oil ratio and water-to-oil ratio and therefore gas        and water content of the upcoming fluid from a well;    -   reduce or eliminate the gas and water cones;    -   reduces the risk of forming areas near the bottomhole zone with        high viscosity fluids;    -   extends the life of the formation and extends the time of its        depletion;    -   increases the index of oil production for a particular formation        or well;    -   increases the stability of oil production;    -   increases the efficiency of gas lift and pumping operations;    -   reduces the pumping electrical energy costs and other costs        associated with oil production;    -   reduces the undesirable washout of sand and other particles from        the formation.

Although the invention herein has been described with respect toparticular embodiments, it is understood that these embodiments aremerely illustrative of the principles and applications of the presentinvention. In particular, the needle of the bottomhole tool may beactivated indirectly by providing a gear reducer between the piston andthe needle body, as well as the spring may be located outside or evenbelow the cylinder. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A method for optimizing oil production rate and overall oil recoveryfrom a formation having an oil well, comprising following steps: a)collecting formation and oil well input data; b) calculating InflowPerformance Relationship curve from said formation and oil well inputdata to describe the projected relationship of a bottomhole formationpressure and an oil production rate; c) identifying a higher than zerodesired value of said bottomhole pressure corresponding to a maximum oilproduction rate from said calculated Inflow Performance Relationshipcurve under current well conditions; d) adjusting the bottomholepressure to the vicinity of said desired bottomhole pressurecorresponding to current well conditions; e) starting oil productionflow; f) monitoring oil well parameters to be within the collectedformation and oil well input data values; g) if deviation of the wellparameters from the collected formation input data is detected,repeating steps (a) through (c) to recalculate the desired value of saidbottomhole pressure; and h) adjusting the bottomhole pressure to saidnewly calculated desired value.
 2. The method as in claim 1, whereinsaid oil well further comprising a bottomhole tool and a wellheadsurface choke, said step (a) includes collecting formation input dataincluding current conditions of said oil well, bottomhole zone, fluidand reservoir parameters, PVT, geometry and dimensions of pipes,bottomhole tool and a wellhead surface choke to populate a mathematicalmodel describing “formation—multi-phase flow—surface choke—bottomholetool” behavior.
 3. The method as in claim 2, wherein said step (d) ofadjusting said bottomhole pressure includes adjusting said bottomholetool.
 4. The method as in claim 3, wherein said step (d) furtherincludes conducting a final adjustment of said bottomhole pressure byadjusting said wellhead surface choke to change the pressure above saidbottomhole tool.
 5. The method as in claim 1, wherein said step (d) ofadjusting said bottomhole pressure is achieved by performing a gas lift.6. The method as in claim 1, further including a step (i) of maintainingsaid bottomhole pressure at a desired level throughout the life of saidwell, whereby maximum overall oil recovery is achieved.
 7. A bottomholetool for adjusting a bottomhole pressure in an oil well containing apipe between a bottomhole zone and a wellhead, said tool comprising: atool housing attached to said pipe in said bottomhole zone of said oilwell, a multi-stage telescopic fluid resistor contained in said toolhousing, a multi-stage needle located inside said telescopic fluidresistor, and a pressure-responsive means to move said needle in and outof said telescopic fluid resistor, said pressure-responsive meansincluding a spring-biased piston attached to said needle and located ina control cylinder attached to said housing, said piston exposed to saidbottomhole pressure above thereof and a pipe pressure below thereof,whereby said needle is maintained at a position defined by a differencebetween said bottomhole pressure and said pipe pressure and said spring,said needle defining with said telescopic fluid resistor a series ofsuccessive annular passages for oil flow therethrough.
 8. The bottomholetool as in claim 7, wherein said multi-stage telescopic flow resistorhas a number of stages equal to same of said multi-stage needle.
 9. Thebottomhole tool as in claim 7, wherein said pipe is sealed against saidwell.
 10. The bottomhole tool as in claim 7, wherein said telescopicfluid resistor having a succession of cylindrical stages with resistordiameters decreasing towards the bottomhole zone of said oil well. 11.The bottomhole tool as in claim 10, wherein said resistor diameters arebetween 2 and 55 mm.
 12. The bottomhole tool as in claim 11, whereinsaid resistor diameters are between 4 and 25 mm.
 13. The bottomhole toolas in claim 10, wherein said multi-stage needle having a succession ofcylindrical stages with needle diameters increasing towards thebottomhole zone of said oil well.
 14. The bottomhole tool as in claim13, wherein said needle diameters are between about 1 and about 50 mm.15. The bottomhole tool as in claim 14, wherein said needle diametersare between about 3 and about 20 mm.
 16. The bottomhole tool as in claim7, wherein the largest diameter of said multi-stage needle is smallerthan the smallest diameter of said telescopic resistor.
 17. Thebottomhole tool as in claim 7 further including a gear reducer betweensaid multi-stage needle and said piston.
 18. A method for optimizing oilproduction rate and overall oil recovery from a formation having an oilwell containing a pipe between a bottomhole zone and a wellhead,comprising following steps: a) providing a bottomhole tool comprising atool housing attached to said pipe in said bottomhole zone of said oilwell, a multi-stage telescopic fluid resistor contained in said toolhousing, a multi-stage needle located inside said telescopic fluidresistor, and a pressure-responsive means to move said needle in and outof said telescopic fluid resistor, said pressure-responsive meansexposed to said bottomhole pressure and a pipe pressure, b) collectingformation and oil well input data; c) calculating Inflow PerformanceRelationship curve from said formation and oil well input data todescribe the projected relationship of a bottomhole formation pressureand an oil production rate; d) identifying a higher than zero desiredvalue of said bottomhole pressure corresponding to a maximum oilproduction rate from said calculated Inflow Performance Relationshipcurve under current well conditions; e) adjusting the bottomholepressure to the vicinity of said desired bottomhole pressurecorresponding to current well conditions; f) starting oil productionflow; g) monitoring oil well parameters to be within the collectedformation and oil well input data values; h) if deviation of the wellparameters from the collected formation input data is detected,repeating steps (a) through (c) to recalculate the desired value of saidbottomhole pressure; and i) adjusting the bottomhole pressure to saidnewly calculated desired value.
 19. The method as in claim 18, whereinsaid step (e) of adjusting said bottomhole pressure includes adjusting apressure at said wellhead to cause a predetermined response thereto ofsaid bottomhole tool to bring said bottomhole pressure to said desiredvalue.